扫描隧道显微术中的微分谱学及其应用

文章介绍了扫描隧道显微术中微分谱学的原理及其在实验中的诸多应用。微分谱（dI／dV谱）和dI／dV成像可用来研究电子局域态密度在能量和空间的分布，即微分谱固定空间一点，反映电子态密度以能量为变量的分布；而dI／dV图像则反映某给定能量的电子局域态密度以空间为变量的分布，二次微分谱（d^2I／dV^2谱）和二次微分成像可以用来反映分子的非弹性隧穿过程，从而研究分子的振动态。     

用LMTO方法对清洁Nb(100)面电子结构的研究

用LMTO-ASA方法计算了铌的体态密度和清洁Nb(100)表面的局域态密度以及有关的表面能。所计算的体态密度与用其它方法计算的态密度符合得很好,表面能则支持了低能电子衍射(LEED)动力学计算的结果,并且通过分析表面局域态密度,提出了清洁Nb(100)表面存在表面态,表面态位于费密能级的上方。 关键词：     

Pushing the envelope to the maximum: Short-period InAs/GaSb type-II superlattices for mid-infrared detectors

Using a newly developed envelope function approximation model that includes interface effects, several InAs/GaSb type-II superlattices (SL) for the 4 μm (around 310 meV) detection threshold were designed. The model predicts that a given threshold can be obtained with progressively thinner SL periods and the thinner designs can have higher mobility and longer Auger lifetime over the thicker designs. The proposed SL structures were grown by molecular-beam epitaxy. The band gaps of SLs determined by low-temperature photoluminescence (PL) remained constant PL peak energy around 340–320 meV with distinctively different designs in the period range from 50.2 to 21.2 Å. Correlation between SL material quality and the full-width at half-maximum (FWHM) of the luminescence peak were made. In situ annealing after SL growth improved surface morphologies and the FWHM of the emission peak for the annealed SL samples were slightly narrower than those of non-annealed SLs.     

Density of states of a terminated superlattice with periodically arrangedδ-defects

Surface electronic structure and the local density of states (LDOS) of a periodicallyδ-doped GaAs/Al _x Ga_1−x As superlattice (SL), terminated by an Al _y Ga_1−y As substrate, are investigated using the wave-function multi-matching procedure within an effectivemass approximation and a Green-function formalism based on factorization and direct techniques. The considered system is described in terms of a semi-infinite Kronig-Penneytype of model interleaved with a periodic array ofδ-function-like potentials, representing deep-center defect sheets. It is shown that the spectrum of surface-localized states and the corresponding space-charge distributions within particular SL minibands can be tailored by varying the terminating conditions (i.e., the substrate parameters) as well as the properties of inserted defects (i.e., the weight and position ofδ-potentials). In this respect, importance of a symmetric/nonsymmetric arrangement ofδ-dopants inside SL barriers and the related effect of miniband crossing/anticrossing are emphasized. Presented at the VIII-th Symposium on Surface Physics, Třešt’ Castle, Czech Republic, June 28 – July 2, 1999. One of us (B.B.) gratefully acknowledges the support of the University of Wrocław via the project for young researchers No. 2314/W/IFD/99, while two of us (R.K. and M.S.) have been supported within the Grant No. 2016/W/IFD/98.     

碳掺杂闭口硼氮纳米管场发射第一性原理研究

运用第一性原理研究了闭口硼氮纳米管（BNNT）顶层掺碳体系（C@BNNT）的电子场发射性能.结果表明：随外电场增强,C@BNNT电子结构变化显著,态密度（DOS）向低能方向移动;碳原子的局域态密度（LDOS）在费米能级附近明显增大;赝能隙、最高占据分子轨道（HOMO）/最低未占据分子轨道（LUMO）能隙减小;体系电荷移向帽端.DOS,HOMO/LUMO及Mulliken电荷分析一致表明,与BNNT相比,C@BNNT电子场发射性能显著改善,且C@BN_moreNT性能更优. 关键词： 碳掺杂 硼氮纳米管 电子场发射 第一性原理     

磁场下含结构缺陷多组分超晶格中的局域电子态和电子输运

在有效质量近似理论下，利用转移矩阵和有效垒高方法研究了有限磁场下含结构缺陷的多组分超晶格中局域电子态的性质.在考虑各组分层有效质量的失配时，外加磁场会导致磁耦合效应的出现.磁耦合效应不仅引起局域电子能级的量子化，并且随着朗道指数或磁场强弱的变化，局域能级及其局域程度都会发生显著移动，特别是对高能区域的局域电子态影响更大.此外，还计算了电子输运系数，讨论了含结构缺陷的三组分超晶格中局域电子能级与输运谱透射禁区中的共振透射峰的关系，发现两者之间有着很好的对应关系，为相应的实验研究提供了依据. 关键词： 超晶格 局域电子态 磁场     

Real-space imaging of alternate localization and extension of quasi-two-dimensional electronic States at graphite surfaces in magnetic fields

We measured the local density of states (LDOS) of a quasi-two-dimensional (2D) electron system near point defects on a surface of highly oriented pyrolytic graphite with scanning tunneling microscopy and spectroscopy. Differential tunnel conductance images taken at very low temperatures and in high magnetic fields show a clear contrast between localized and extended spatial distributions of the LDOS at the valley and peak energies of the Landau level spectrum, respectively. The localized electronic state has a single circular distribution around the defects with a radius comparable to the magnetic length. The localized LDOS is in good agreement with a spatial distribution of a calculated wave function for a single electron in 2D in a Coulomb potential in magnetic fields.     

STM/STS measurements of two-dimensional electronic states trapped around surface defects in magnetic fields

The local density of states (LDOS) near point defects on a surface of highly oriented pyrolytic graphite (HOPG) was studied at very low temperatures in magnetic fields up to 6 T. We observed localized electronic states over a distance of the magnetic length around the defects in differential tunnel conductance images at the valley energies of the Landau levels (LLs) as well as relatively extended states at the peak ones of LLs. These states appear mainly at energies above the Fermi energy corresponding to the electron LL bands. The data suggest that the quantum Hall state is realized in the quasi two dimensional electron system in HOPG. At the peak energy associated with the n=0 (electron) and -1 (hole) LLs characteristic of the graphite structure, a reduced LDOS around the defects is observed. The spatial distribution is almost field independent, which indicates that it represents the potential shape produced by the defects.     

Interplay between edge states and simple bulk defects in graphene nanoribbons

We study the interplay between the edge states and a single impurity in a zigzag graphene nanoribbon. We use tight-binding exact diagonalization techniques, as well as density functional theory calculations to obtain the eigenvalue spectrum, the eigenfunctions, as well as the dependence of the local density of states (LDOS) on energy and position. We study the strictly zero-energy eigenfunctions using symmetry considerations, as well as tight-binding techniques. Moreover, we note that roughly half of the unperturbed eigenstates in the spectrum of the finite-size ribbon hybridize with the impurity state, and the corresponding eigenvalues are shifted with respect to their unperturbed values. The maximum shift and hybridization occur for a state whose energy is inverse proportional to the impurity potential, and give rise to an impurity peak in the DOS spectrum. We find that the interference between the impurity and the edge gives rise to peculiar modifications of the LDOS of the nanoribbon, in particular to oscillations of the edge LDOS. These effects depend on the size of the system, and decay with the distance between the edge and the impurity.     

Impurity cyclotron resonance in InGaAs/GaAs superlattice and InGaAs/AlAs superlattice grown on GaAs substrates

Various temperature measurements of cyclotron resonance (CR) under pulsed ultra-high magnetic field up to 160 T were carried out in InGaAs/GaAs superlattice (SL) and InGaAs/AlAs SL samples grown by molecular beam epitaxy on GaAs substrates. Clear free-electron CR and impurity CR signals were observed in transmission of CO₂ laser with wavelength of 10.6 μm. A binding energy of impurities in these SLs was roughly estimated based on the experiment as result, and we found it was smaller than the previous experimental result of GaAs/AlAs SLs and theoretical calculation with a simple model.     

Local density of states and order parameter configurations in layered ferromagnet-superconductor structures

We analyze the local density of states (LDOS) of heterostructures consisting of alternating ferromagnet, F, and superconductor, S, layers. We consider structures of the SFS and SFSFSFS type, with thin nanometer scale F and S layers, within the ballistic regime. The spin-splitting effects of the ferromagnet and the mutual coupling between the S regions, yield several nontrivial stable and metastable pair amplitude configurations, and we find that the details of the spatial behavior of the pair amplitude govern the calculated electronic spectra. These are reflected in discernible signatures of the LDOS. The roles that the magnetic exchange energy, interface scattering strength, and the Fermi wavevector mismatch each have on the LDOS for the different allowed junction configurations, are systematically investigated.     

Radiative and non-radiative local density of states on disordered plasmonic films

We present numerical calculations of the local density of optical states (LDOS) in the near field of disordered plasmonic films. The calculations are based on an integral volume method, that takes into account polarization and retardation effects, and allows us to discriminate radiative and non-radiative contributions to the LDOS. At short distance, the LDOS fluctuations are dominated by non-radiative channels, showing that changes in the spontaneous dynamics of dipole emitters are driven by non-radiative coupling to plasmon modes. Maps of radiative and non-radiative LDOS exhibit strong fluctuations, but with substantially different spatial distributions.     

氮掺杂及水分子吸附碳纳米管电子场发射第一性原理研究

对闭口碳纳米管(CNT)顶端分层掺氮及吸附不同数目水分子体系,运用第一性原理研究了有电场存在时的电子场发射性能.结果表明：掺氮并吸附水分子的CNT结构稳定;外电场愈强、水分子数愈多,体系态密度(DOS)向低能端移动幅度愈大且最高分子占据轨道(HOMO)/最低分子空轨道(LUMO)能隙愈小.吸附能,DOS/LDOS,HOMO/LUMO及其能隙分析一致表明,第三层氮掺杂CNT吸附不同数目水分子体系的场发射性能最佳. 关键词： 氮掺杂 水吸附 密度泛函理论 电子场发射     

Spatial imaging of two-dimensional electronic states in semiconductor quantum wells

We measure the spatial distribution of the local density of states (LDOS) at cleaved surfaces of InAs/GaSb isolated quantum wells and double quantum wells (DQWs) by low-temperature scanning tunneling spectroscopy. Distinct standing wave patterns of LDOS corresponding to subbands are observed. These LDOS patterns and subband energies agree remarkably well with simple calculations with tip-induced band bending. Furthermore, for the DQWs, coupling of electronic states between the quantum wells is also clearly observed.     

利用扫描隧道谱(STS)对石墨单晶表面局域电子态的研究

利用ＳＴＳ测量并结合扫描隧道显微镜（ＳＴＭ）扫描图象，给出一组沿石墨单晶表面原子分辨的ＳＴＭ图象上某一线段各点处的扫描隧道谱．ｄ（lｎI）／ｄ（lｎＶ）～ｅＶ由测量谱给出的样品表面Ｅ_Ｆ附近局域态密度分布与由体能带结构计算得到的结果在一定程度上相符合，将各条曲线中Ｅ_Ｆ附近的态密度峰能量对相应的空间位置作图，给出石墨表面E_F附近能态密度在测量区域内实空间的变化。通过对表面不等价Ａ，Ｂ类原子处局域电子结构的分析并利用简单模型进行计算，给出了与实验 关键词：     

Optimization of InAs/GaSb type-II superlattices for high performance of photodetectors

The optimum growth conditions and strain balancing processes have been studied using molecular beam epitaxy (MBE) grown 51 Å InAs/40 Å GaSb type-II superlattices (SLs) designed to have cut-off wavelength of 10 μm. The most dominant factor in reducing the defect level in the SL structure was buffer growth temperature evidenced by transmission electron microscopy. In the study of the strain balancing process, the SLs could be lattice matched to the GaSb substrate by increasing the thickness of the InSb interfaces (IFs) from a nominal value of 1.0 to 1.4 ML, however, the structural quality degraded dramatically when the thickness of IFs reached beyond 1.0 ML. By optimizing the growth condition and MBE shutter sequences, micron thick InAs/GaSb SLs with a reduced lattice mismatch were routinely obtained with the full-width half-maximum of 18 arcsec, and the root mean square values of surface roughness of 2 Å in 5 μm area scan of atomic force microscopy demonstrating high quality. Correlation between material quality and photoresponse signal strength in photoconductivity measurements was made on SL samples with cut-off wavelength on the order of 10 μm.     

Spatial variation in the electronic structures of carpetlike graphene nanoribbons and sheets

Graphene, when deposited on a supporting substrate with a step edge, may be deformed in the presence of the step edges of the substrate. In this study, we have investigated a spatial variation in the local electronic structure near the step region, by performing first-principles calculations for carpetlike armchair graphene nanoribbons (C-AGNR) and two-dimensional periodic carpetlike graphene sheets (PCGS). Our results indicate no practical difference in the local density of states (LDOS) between those of flat and step regions. Interestingly, however, the PCGS shows a remarkable variation in the LDOS with an external electric field (E-field). Furthermore, we also discuss the dependence of the direction and the magnitude of the applied E-field on the spatial variation in the LDOS.     

掺杂GaN/AlN超晶格第一性原理计算研究

第一性原理计算方法在解释实验现象和预测新材料结构及其性质上有着重要作用. 因此, 通过基于密度泛函理论的第一性原理的方法, 本文系统地研究了Mg和Si掺杂闪锌矿和纤锌矿两种晶体结构的GaN/AlN超晶格体系中的能量稳定性以及电学性质. 结果表明: 在势阱层(GaN 层)中, 掺杂原子在体系中的掺杂形成能不随掺杂位置的变化而发生变化, 在势垒层(AlN层)中也是类似的情况, 这表明对于掺杂原子来说, 替代势垒层(或势阱层)中的任意阳离子都是等同的; 然而, 相比势阱层和势垒层的掺杂形成能却有很大的不同, 并且势阱层的掺杂形成能远低于势垒层的掺杂形成能, 即掺杂元素(Mg_Ga, Mg_Al, Si_Ga和Si_Al)在势阱区域的形成能更低, 这表明杂质原子更易掺杂于结构的势阱层中. 此外, 闪锌矿更低的形成能表明: 闪锌矿结构的超晶格体系比纤锌矿结构的超晶格体系更易于实现掺杂; 其中, 闪锌矿结构中, 负的形成能表明: 当Mg原子掺入闪锌矿结构的势阱层中会自发引起缺陷. 由此, 制备以闪锌矿结构超晶格体系为基底的p型半导体超晶格比制备n型半导体超晶格需要的能量更低并且更为容易制备. 对于纤锌矿体系来说, 制备p型和n型半导体的难易程度基本相同. 电子态密度对掺杂体系的稳定性和电学性质进一步分析发现, 掺杂均使得体系的带隙减小, 掺杂前后仍然为第一类半导体. 综上所述, 本文内容为当前实验中关于纤锌矿结构难以实现p型掺杂问题提供了一种新的技术思路, 即可通过调控相结构实现其p型掺杂.     

Type-II InAs/GaSb superlattices for very long wavelength infrared detectors

Type-II superlattices (SLs) can be designed for semiconductor band gaps as large as 400 meV down to semimetallic. This flexibility in design makes them an excellent candidate for infrared photodiodes with cut-off wavelengths beyond 15 μm. There are relatively few options for high-performance infrared detectors to cover wavelengths longer than 15 μm, especially for operating temperatures above 15 K. In the past few years, excellent results have been obtained on photoconductive and photodiode samples designed for infrared detection in the very long wavelength infrared (VLWIR) range (λ>15 μm). There is a variety of possible designs for these SLs which will produce the same narrow band gap by adjusting individual layer thicknesses, or indium content, in the InGaSb layer. Several of these different design options have been grown and characterized. These designs often require monolayer control per layer over hundreds of repeats in the SL. Photoresponse spectra for type-II SLs are compared to show how the design choices not only change the band gap but also the band structure, as reflected in features observed in the spectra. Theoretical modeling results are used to interpret the photoresponse spectra. SLs with cut-off wavelengths ranging from 15 to 25 μm are covered.     

Calculation of coverage dependence of electronic density of states and binding energy for chemisorbed oxygen on Ni(001)

We present a tight-binding calculation of the electronic local density of states (LDOS) and binding energy for chemisorbed oxygen on a Ni(001) surface at various coverages. The electronic structure is determined by treating explicitly the 4s and 3d electrons and their hybridization for the substrate Ni atoms and the oxygen 2p electrons. The continued-fraction technique is used to determine the LDOS. The obtained LDOS for the p(2 × 2) and c(2 × 2) structures is in good agreement with photoemission measurements. Our calculation shows that with increasing oxygen coverage, the binding energy of oxygen decreases, whereas the charge transfer on oxygen increases. These results are discussed in connection with oxidation of the surface nickel occuring at high coverages.